A number of curriculums (Bob Jones University, Robinson, Alpha Omega, Switched-On Schoolhouse, etc.) offer placement tests, which should give you a general idea where your son is, at least compared to their grade level systems. This is the best choice if you want to go with a specific curriculum.

Scroll down to the Elementary School Resources section and click the individual subjects and then the tests corresponding to your son's assumed grade level (or below, if he can't handle the material at that level). This will also tell you where your son is and what material he needs to cover to catch up.

I am going to start homeschooling my 7 year old this coming school year. She has breezed through kindergarden and 1st grade with straight "O's" almost effortlessly and is reading at a third grade reading level. Because of her birthday (Oct.), she was one of the oldest children in her class. Would it be safe to just purchase a third grade curriculum (at least in language arts)? Is placement testing really necessary and this age, or can I just get the placement info from her former teacher?

Grade level content varies from curriculum to curriculum, so if you're transitioning from public school and there are placement tests available for the curriculum you want, it's probably a good idea to take a few days and do one. Even if you find out you made the correct choice originally, the placement test won't hurt, since it will highlight the areas your daughter is weakest in.